Biology Reference
In-Depth Information
infrastructure and freshwater aquifers free from the invasion of salt water,
has coincided with a rise in sea level. This rise is projected to increase in
the centuries ahead to a maximum of about 80 m if the Antarctic ice sheet
(7.7 million cubic miles of ice, currently losing about 36 cubic miles of
ice each year) and the Greenland ice sheet (680,000 cubic miles of ice)
melt completely with continued global warming. Already there has been
a 20 percent increase in intense-level fl oods in the past few decades,
prompting articles in the popular media asking, “Are we losing Louisi-
ana?” and similar questions about the Antarctic icecap, the Arctic coast-
lines, Bangladesh, and other places. The answer from the present is yes.
The answer from the past is yes—and it has happened repeatedly, and the
evidence has long been available from geologic deposits, soil profi les, and
historical records.
PALEOTEMPERATURES
The technique for determining temperatures of the distant past is based
on measurement of the isotopes
16
O and
18
O in marine waters. Both are
part of the CO
2
incorporated into the calcite and silicate shells of marine
invertebrates, for example, foraminifera, marine gastropods, radiolarians
(fi g. 3.2). The ratio of the isotopes in the shells changes with temperature
(more
18
O is taken up as the water cools). Curves have been constructed
from Deep Sea Drilling Project (DSDP) cores drilled in all the world's
oceans by research vessels such as the Glomar
Challenger
(fi g. 3.3) as re-
counted by Kenneth Hsü in
Challenger at Sea
(1992). Calibration of the ra-
tios to specifi c temperatures is based on ecological studies of modern spe-
cies similar to those in the core, and on aquaria experiments. The calcium
carbonate-oxygen isotope geothermometer is now the most widely used
tool for estimating ancient ocean temperatures. Fossil fl oras and faunas and
geologic events can be plotted on the curve to provide a temperature prov-
enance for both marine and adjacent terrestrial biota (fi g. 3.4).
A complication arose in calculating temperatures from the isotopes be-
cause as water evaporates from the ocean surface, more of the lighter
16
O
is distilled, potentially affecting the
16
O/
18
O ratio in the basin. During non-
glacial times, the water is rapidly returned by river fl ow and groundwater
seepage, circulated by marine currents, and the ratio remains relatively
constant. In glacial times, however, the disproportionate amount of
16
O in
the evaporated water is retained in the ice for long periods of time, and the
ratio in marine basins is altered by a factor other than temperature (i.e., ice
volume). If there is agreement about the presence of glaciers during a given
